1. Field of the Invention
The invention relates generally to the field of computer systems and more particularly to 3D computer graphics processor.
2. Description of Related Art
In 3D computer graphics, the view frustum is the region of a three dimensional space in the modeled world that may appear on a computer display screen represented by the 2D screen space (hereinafter also referred to as the screen space). The exact shape of the view frustum may vary, e.g. it may be a rectangular cube, a rectangular pyramid, or other suitable geometric shape. In an example, the frustum may be formed with six planes: near plane, far plane, +X plane, −X plane, +Y plane, and −Y plane. The planes that cut the view frustum perpendicular to the viewing direction are called the near plane and the far plane, which may also be called the near Z plane and the far Z plane.
3D objects in the view frustum may be represented by multiple geometric primitives, such as polygons, triangles, line segments, points, or other appropriate geometric elements. Geometric primitives that fall outside of the view frustum will not be visible and may be discarded, removed, or abandoned, in view frustum clipping, or culling, operation of the graphics processing pipeline. Here, culling refers to the complete removal of a geometric primitive which is completely outside of the view frustum while clipping refers to the partial removal of a geometric primitive which has a partial portion outside of the view frustum. Culling may be considered as a special case of clipping where the partial portion removed defaults to the complete geometric primitive. Clipping/culling 3D objects outside of the view frustum is not necessary to achieve a correct image output, but it accelerates the rendering process by eliminating the unneeded rasterization and post-processing on primitives that will not appear on the computer display screen anyway. Graphics processing pipeline typically includes a primitive assembly module (hereinafter referred to as the PA module) where 3D objects, which may be represented by multiple vertices computed from a vertex shader module, are converted to geometric primitives such as triangles, lines, points, polygons, or other appropriate geometric elements. For example, lines may be drawn from a starting-point (or starting-vertex) to an ending-point (or an ending-vertex) in order to pass conformance tests. The reason is that the ending-vertex may or may not be part of the line. PA module may also perform a conversion from the view frustum to the 2D screen space. In some examples, only the X and Y coordinates are transformed into screen space, while the per-pixel Z values may be stored in a Z-buffer. After processed by the PA module, the primitives are in the screen space. The rest of the pipe acts only on the screen space. In some examples, a clipping block may be added behind a primitive assembly block and before the SE module. In this case, the primitive assembly block would just assemble the primitives, the clipper block would clip the primitives and generate more primitives based on this clipping and than pass it on to a transformation pipe which transforms the clipped primitives to the screen space before it enters the SE module. This whole pipe including the primitive assembly block, clipper block, and the transformation pipe is called the PA module. Graphics processing pipelines typically also include a rasterization module (hereinafter referred to as the RA module) where all the pixels in a clipping window inside the 2D screen space are rendered. The RA module operates on the clipping window which may be a portion of the view frustum. Pixels in the screen space outside of the clipping window also need to be clipped/culled, i.e., discarded, removed, or abandoned. In addition to the primitive assembly module and the rasterization module, there may also be additional set up functions (e.g., computing an initial point of rasterization for a primitive) performed in a set up module (hereinafter referred to as the SE module). In the prior art, clipping/culling of 3D objects outside of the view frustum is typically done in a single place inside a graphics processing pipeline. Such an implementation can cause a bottleneck in the pipeline because of the large number of computation required for performing clipping/culling. For an example, view frustum clipping/culling may be done in the screen space. In this example, all the 3D objects are transformed to screen space and sent to the setup engine to do the clipping according to the clipping window. The advantage of this approach is that the hardware in the PA module may be very simple. The disadvantage is that all the 3D objects need to be transformed to screen space, even the objects outside of view frustum or clipping window. This may increase complexity and cost of the graphics processing pipeline implementation. In another example, all 6-plane clipping may be done in the view frustum before the conversion to the 2D screen space. The advantage of this approach is that all polygons outside of the view frustum are clipped; only the polygons inside the view frustum are transformed to screen space and sent to setup engine. The disadvantage of this approach is that the algorithm of doing 6 plane clipping is complex and expensive for hardware implementation.